1. Field of the Invention
The present invention relates to the field of gas turbines, and particularly to the field of turbine engines for aircraft such as helicopters.
The present invention relates more particularly to a turbine engine, in particular for a helicopter, the engine including a gas generator and a free turbine driven in rotation by the gas flow generated by the gas generator.
2. Description of the Related Art
Conventionally, the gas generator comprises at least one centrifugal compressor and a turbine that are coupled together in rotation. Operation is as follows: the cool air entering the turbine engine is compressed due to the rotation of the compressor before being sent to a combustion chamber in which it is mixed with a fuel. The burnt gas from the combustion is then exhausted at high speed towards the turbine of the gas generator.
A first expansion then takes place in the turbine of the gas generator, during which expansion said turbine extracts the energy necessary for driving the compressor.
The turbine of the gas generator does not absorb all of the energy of the burnt gas and the surplus energy constitutes the gas flow generated by the gas generator.
Said gas generator therefore provides kinetic energy to the free turbine so that a second expansion takes place in the free turbine which transforms the energy from the gas into rotational kinetic energy in order to drive a receiver unit, such as the rotor of the helicopter.
Naturally, the turbine engine is intended to operate within set limits, and the turbine engine is kept within such limits by acting mainly upon the flow rate of the fuel injected into the combustion chamber.
Thus, during a stage of turbine engine acceleration, in particular in flight, following a power demand from the pilot, the flow rate of the fuel injected into the combustion chamber is increased, and that has the effect of increasing the generated gas flow and consequently of increasing the power delivered to the free turbine.
However, acceleration must take place within certain limits in order to avoid the surge phenomenon that is damaging to the turbine engine. This phenomenon can take place during acceleration that is too sudden and during which, because of a fuel flow rate that is too high, the pressure downstream from the combustion chamber becomes higher than the pressure upstream, i.e. the pressure of the compressed air delivered by the compressor. Under such circumstances, the first expansion takes place not only downstream, but also upstream so that the flow rate of the burnt gas becomes zero and the pressure in the compressor drops.
It is well known that the surge phenomenon may have detrimental consequences on the parts constituting the turbine engine and on the power delivered by the turbine engine.
As a transient stage of acceleration requires a significant increase in the fuel flow rate, a margin (known as a surge margin) is generally provided that is large enough for the turbine engine to operate without surging in its operating range.
It can thus be understood that the acceleration capacity of such a turbine engine is limited by its surge margin.